Pesticide toxicity to bees

Pesticides vary in their effects on bees. Contact pesticides are usually sprayed on plants and can kill bees when they crawl over sprayed surfaces of plants or other areas around it. Systemic pesticides, on the other hand, are usually incorporated into the soil or onto seeds and move up into the stem, leaves, nectar, and pollen of plants.[1]

Of contact pesticides, dust and wettable powder pesticides tend to be more hazardous to bees than solutions or emulsifiable concentrates. When a bee comes in contact with pesticides while foraging, the bee may die immediately without returning to the hive. In this case, the queen bee, brood, and nurse bees are not contaminated and the colony survives. Alternatively, the bee may come into contact with an insecticide and transport it back to the colony in contaminated pollen or nectar or on its body, potentially causing widespread colony death.[2]

Actual damage to bee populations is a function of toxicity and exposure of the compound, in combination with the mode of application. A systemic pesticide, which is incorporated into the soil or coated on seeds, may kill soil-dwelling insects, such as grubs or mole crickets as well as other insects, including bees, that are exposed to the leaves, fruits, pollen, and nectar of the treated plants.[3][4]

Pesticides are linked to Colony Collapse Disorder and are now considered a main cause, and the toxic effects of Neonicotinoids on bees are confirmed.[5] Currently, many studies are being conducted to further understand the toxic effects of pesticides on bees. Agencies such as the EPA[6] and EFSA[7] are making action plans to protect bee health in response to calls from scientists and the public to ban or limit the use of the pesticides with confirmed toxicity.

Insecticide toxicity is generally measured using acute contact toxicity values LD50 – the exposure level that causes 50% of the population exposed to die. Toxicity thresholds are generally set at[8][9]

The acute toxicity of pesticides on bees, which could be by contact or ingestion, is usually quantified by LD50. Acute toxicity of pesticides causes a range of effects on bees, which can include agitation, vomiting, wing paralysis, arching of the abdomen similar to sting reflex, and uncoordinated movement. Some pesticides, including Neonicotinoids, are more toxic to bees and cause acute symptoms with lower doses compared to older classes of insecticides. Acute toxicity may depend on the mode of exposure, for instance, many pesticides cause toxic effects by contact while Neonicotinoids are more toxic when consumed orally. The acute toxicity, although more lethal, is less common than sub-lethal toxicity or cumulative effects.[10][11]

Field exposure of bees to pesticides, especially with relation to neonicotinoids, is most commonly sub-lethal. Sub-lethal effects to honey bees are of major concern and include behavioral disruptions such as disorientation, reduced foraging, impaired memory and learning, and a shift in communication behaviors. Additional sub-lethal effects may include compromised immunity of bees and delayed development.[10]

Neonicotinoids are especially likely to cause cumulative effects on bees due to their mechanism of function as this pesticide group works by binding to nicotinic acetylcholine receptors in the brains of the insects, and such receptors are particularly abundant in bees. Over-accumulation of acetylcholine results in paralysis and death.[10]

Colony collapse disorder (CCD) is a syndrome that is characterized by the sudden loss of adult bees from the hive. Many possible explanations for CCD have been proposed, but no one primary cause has been found. The US Department of Agriculture (USDA) has indicated in a report to Congress that a combination of factors may be causing CCD, including pesticides, pathogens, and parasites, all of which have been found at high levels in affected bee hives.[12]

Colony Collapse Disorder has more implication than the extinction of some bee species; the disappearance of honeybees can cause catastrophic health and financial impacts. One mouthful in three of the food we eat may depend directly or indirectly on pollination by honeybee. Honeybee pollination has an estimated value of more than $14 billion annually to the United States agriculture. Honeybees are required for pollinating many crops, which range from nuts to vegetables and fruits, that are necessary for human and animal diet.[13]

The EPA updated their guidance for assessing pesticide risks to honeybees in 2014. For the EPA, when certain pesticide use patterns or triggers are met, current test requirements include the honey bee acute contact toxicity test, the honey bee toxicity of residues on foliage test, and field testing for pollinators. EPA guidelines have not been developed for chronic or acute oral toxicity to adult or larval honey bees. On the other hand, the PMRA (Pest Management Regulatory Agency) requires both acute oral and contact honey bee adult toxicity studies when there is potential for exposure for insect pollinators. Primary measurement endpoint derived from the acute oral and acute contact toxicity studies is the median lethal dose for 50% of the organisms tested (i.e., LD50), and if any biological effects and abnormal responses appear, including sub-lethal effects, other than the mortality, it should be reported.

The EPA's testing requirements do not account for sub-lethal effects to bees or effects on brood or larvae. Their testing requirements are also not designed to determine effects in bees from exposure to systemic pesticides. With colony collapse disorder, whole hive tests in the field are needed in order to determine the effects of a pesticide on bee colonies. To date, there are very few scientifically valid whole hive studies that can be used to determine the effects of pesticides on bee colonies because the interpretation of such whole-colony effects studies is very complex and relies on comprehensive considerations of whether adverse effects are likely to occur at the colony level.[14]

A March 2012 study[15] conducted in Europe, in which minuscule electronic localization devices were fixed on bees, has shown that, even with very low levels of pesticide in the bee's diet, a high proportion of bees (more than one third) suffers from orientation disorder and is unable to come back to the hive. The pesticide concentration was order of magnitudes smaller than the lethal dose used in the pesticide's current use. The pesticide under study, brand-named "Cruiser" in Europe (thiamethoxam, a neonicotinoid insecticide), although allowed in France by annually renewed exceptional authorization, could be banned in the coming years by the European Commission.

Bees poisoned with carbaryl can take 2–3 days to die, appearing inactive as if cold. Sevin should never be sprayed on flowering crops, especially if bees are active and the crop requires pollination. Less toxic formulations exist.

Malathion is highly toxic to bees and other beneficial insects, some fish, and other aquatic life. Malathion is moderately toxic to other fish and birds, and is considered low in toxicity to mammals.[41]

safened by repellency under arid conditions. Permethrin is also the active ingredient in insecticides used against the Small hive beetle, which is a parasite of the beehive in the temperate climate regions.

Clothianidin is a major metabolite of Thiamethoxam. A two-year study published in 2012 showed the presence of clothianidin and thiamethoxam in bees found dead in and around hives situated near agricultural fields. Other bees at the hives exhibited tremors and uncoordinated movement and convulsions, all signs of insecticide poisoning.[66]

Neonicotinoids are one of the leading suspected causes of colony collapse disorder in honey bees. The specific causes are unclear, however, there has been some research to show that neonicotinoids have deleterious health effects on colony queens. Managed honeybee colonies are colonies that are “man-made.” That is, they are not naturally occurring. Rather they are raised and rented out to farmers.[69]

Decline of annual honey production between 2000 and 2011 by one third from 221 million to 148 million pounds with increased Neonicotinoids use in the United States.

There is some controversy surrounding the specific issue of whether or not neonicotinoids actually do negatively affect managed honeybee colonies. Perhaps one of the most popular studies showing a significant association between colony collapse disorder and neonicotinoids is 'Sub-lethal exposure to neonicotinoids impaired honeybees Winterization before proceeding to colony collapse disorder' by Chensheng Lu.[70] Chengsheng became somewhat of a folk hero among environmental activists after his Harvard study was published, however, there has been some dissent from bee researchers in Australia,[71] Canada,[72] and even the USDA.[73] That is not to say that Mr. Lu’s findings are not accurate. The problem is that there simply is not a consensus yet on the real association between neonicotinoids and colony collapse disorder.

Lindane, BHC (banned in California).[79] Lindane was also denied re-registration for agricultural use in the US by the EPA in 2006[80]

EPA Proposal to Protect Bees from Acutely Toxic Pesticides in the US[edit]

The EPA is proposing to prohibit the application of certain pesticides and herbicides known toxic to bees during pollination periods when crops are in bloom. Growers routinely contract with honeybee keepers to bring in bees to pollinate their crops that require insect pollination. Bees are typically present during the period the crops are in bloom. Application of pesticides during this period can significantly affect the health of bees. These restrictions are expected to reduce the likelihood of high levels of pesticide exposure and mortality for bees providing pollination services. Moreover, the EPA believes these additional measures to protect bees providing pollination services will protect other pollinators as well.[81]

The proposed restrictions would apply to all products that have liquid or dust formulations as applied, foliar use (applying pesticides directly to crop leaves) directions for use on crops, and active ingredients that have been determined via testing to have high toxicity for bees (less than 11 micrograms per bee). These restrictions would not replace already existing more restrictive, chemical-specific, and bee-protective provisions. Additionally, the proposed label restrictions would not apply to applications made in support of a government-declared public health response, such as use for wide area mosquito control. There would be no other exceptions to these proposed restrictions.[81]

Avoiding pesticide application directly to blooming flowers as much as possible can help limit the exposure of honeybees to toxic materials as honeybees are attracted to all types of blooming flowers. If blooming flowers must be sprayed with pesticides for any reason, they should be sprayed in the evening or night hours as bees are not in the field at that time. Usual foraging hours of honeybees are when the temperature is above 55-60 °F during the daytime, and by the evening, the bees return to the hives.